The present invention is concerned with a particle separator for the separation of particulate mixtures comprising species that exhibit difference in electrical conductivity and, more particularly, with the separation of particulate mixtures comprising species that exhibit difference in electrical conductivity through electrostatic separation.
Mineral separation plants used in the titanium mineral processing industry world-wide consist essentially of similar process technologies applied in a manner that is often tailored to an individual ore bodies separation requirements. Dependent upon a wide number of factors including particle size and shape, mineral grade, geology of the ore body, type of mineral species present and the physical characteristics of said mineral species, a unique recovery process is applied to optimise plant performance and satisfy operational and capital cost targets. Nevertheless, all titanium mineral processing plants in the world utilise similar process technologies applied in varying ways to accomplish their process needs.
Mining is carried out by firstly excavating the ore and subjecting it to gravity concentration which isolates the heaviest particles into what is termed a heavy mineral concentrate. The heavy mineral concentrates are sent to a dry separation plant, where individual minerals species (of which there may up to 20 or more present) are separated using their different magnetic, electrical or other physical properties, often at elevated temperatures. Separation equipment commonly includes but is not limited to, high-tension electrostatic roll (HTR) and electrostatic plate (ESP) separators, as well as gravity and magnetic processes. Using electrostatic separation techniques the conductors such as rutile and ilmenite are separated from the non-conductors such as zircon, quartz and monazite. These separators are extensively used for the separation of conductor and non-conductor mineral species typically found in the titanium minerals industry.
A wide variety of electrostatic induced charge and ionised field separators have been invented over the last 90 years however the devices of existing commercial designs described below have undergone little fundamental change in recent years.
Based on the charging mechanisms employed, three basic types of xe2x80x9celectrostaticxe2x80x9d separators include; (1) high tension roll ionised field separators (HTR), (2) electrostatic plate and screen static field separators (ESP and ESS herein called ESP) and (3) triboelectric separators. ESP and HTR separators are the most commonly used today although in recent times some interest has been directed towards triboelectric separators however their application remains limited to mineral species that can be contact charged and so they are suitable for separations of non-conductor species only.
Customarily, HTR separators utilise a grounded roll that transports the feed material through the high voltage ionising field (corona) which charges the particles by ion bombardment. Conducting particles lose their charge to the earthed roll and are thrown from the roll by centrifugal and gravity forces. Non-conducting particles are pinned to the rotor and are transported further around the roll before their charge either dissipates and they are thrown off or are removed by either mechanical means (brush) or high voltage AC wiper.
ESP separators have an electrode designed to generate a static field and the particles are charged by conductive induction. In their common form ESP separators utilise a stationary grounded surface such as a plate over which the material flows, forming the connection to ground that particles must have to allow them to become charged by induction. Triboelectric separators do not use the electric field to effect particle charging. Particle to particle and/or particle to surface charging occurs when particle species with different contact charging potential are brought into contact with one another. The particle charge attained can then be utilised to effect a separation in a static electric field.
These three basic separation types are often not present alone in any mechanism and the machine characterisation essentially refers to the predominant or major separating effect. The present invention relies primarily on conductive induction to charge the particles and so the operation of an ESP separator is described in more detail below.
ESP conductive induction separators customarily comprise a curved, inclined electrically grounded plate onto and over which a feed mixture comprising species which differ in their electrical conductivity (some being conducting species or xe2x80x9cconductorsxe2x80x9d and other being non-conducting species or xe2x80x9cnon-conductorsxe2x80x9d) flows. The mixture is discharged onto this plate usually from a feed chute so that it travels over the plate due to gravity and in electrical contact with the plate surface.
The plate extends beside and below a high voltage electrode spanning the full width of the separation zone. The grounded plate is commonly curved, convex or xe2x80x9cSxe2x80x9d shaped, which provides good particle to plate contact when clean. Particles flowing over the grounded plate pass though the high potential electric field produced between the electrode and the plate itself whereupon a charge is induced into the conductive particles. These conductive particles acquire a charge of opposite polarity to the electrode whereas the non-conductive particles remain uncharged. The charged conductors are lifted off the grounded plate due to the physical attraction of oppositely charged bodies and are attracted towards the electrode.
Thus the conductors lift away from a gravitationally induced trajectory before falling through a splitter type collection means below and/or beyond the plates lower edge dividing the feed into a mainly conductor and a mainly non-conductor fraction.
The above description of the mechanism describes a one-stage separation process. Electrostatic Plate Separators (ESP) typically would incorporate 5 identical stages with up to two starts or individual streams being treated in one machine. Each new stage follows the last with material cascading from one stage to the next. Typically, conductors are gradually removed from the non-conductors whom continue on to the next stage for re-treatment.
Each stage is similar to the first with feed chute, grounded plate, electrode and splitter system duplicated and arranged one above the other in a vertical configuration. Adjustment of the splitters, electrode position and feed plate angle is typically done at each stage independently of other stages.
In the treatment of mixtures of particles with a range of physical characteristics including conductivity and particle size and density, it is necessary to adjust the relative positions of the feed plate, electrode and splitters to optimise the separation. It is usually necessary to adjust not only the air gap between plate and electrode but also the slope and shape of the plate and splitter positions independently on each stage. Voltage and polarity is traditionally similar over all starts and stages on each machine bank as a single high voltage power supply is used for simplicity reasons. A typical process in a plant may utilise many of these machines installed side by side or otherwise and if operating on the same duty, the operators would normally aim to set up electrode and splitter settings similarly for each machine.
There have also been proposals to use other than a grounded flat or curved plate, for example U.S. Pat. No. 2,258,767 discloses an electrostatic separating apparatus in which the grounded plate is formed into a roll. The roll rotates continuously and a portion thereof comes into appropriate juxtaposition with an electrode for conductive induction of charge in conducting species located on the surface of the roll. The invention is characterised in that the electrode is a rotatable cylindrical electrode and includes wiper means containing abrasive material co-operating with said electrode for polishing it as it rotates. At column 1 lines 16-26, it is said to be desirable to have a smooth surface on the high potential electrode in order to prevent the piling up of particles thereon, thereby forming discharge points and causing arcing to take place between the electrodes. The wiper employed in U.S. Pat. No. 2,258,767 achieves this by polishing the electrode surface and removing therefrom any particles which might tend to form discharge points. An angle is provided for guiding the material to be separated onto the roll, and this angle includes a swatch of material of the same kind as that used to polish the electrode to seal the gap between the angle and the roll. The purpose of the seal is to ensure that particulate material does not escape through the gap between the angle and the roll. The material that seals the gap seems not to act upon the separation roll and is apparently specified as being made from the same material as the wiper for the electrode on the basis that commonality of materials is good design practice in manufacturing such devices in order to reduce cost and facilitate provision of replacement parts. There is no discussion in U.S. Pat. No. 2,258,767 of any reduction in conductivity over time through surface contamination of the roll nor of any means for ameliorating this loss.
According to a first aspect of the present invention there is provided a particle separator for the separation of particulate mixtures comprising species that exhibit difference in electrical conductivity, comprising:
a conductive surface including a separation zone over which said species move at a predetermined velocity, the electrical conductivity of which is reduced over time through surface contamination;
feeder means for feeding said species to said separation zone;
an electrode arrangement spaced apart from said conductive surface and capable of inducing charge in conducting species as they move across said separation zone and lifting said conducting species from said conductive surface once charged;
cleaning means located other than within said separation zone for producing a conductive surface free from surface contamination; and
drive means for movement of said conductive surface relative to said electrode arrangement in order to bring said conductive surface free from surface contamination into a position where it constitutes said separation zone.
Advantageously said cleaning means is spaced apart over said conductive surface from said feeder means.
Typically said feeder means feeds said species to an upper portion of said conductive surface and said species move downwardly through said separation zone. In this arrangement said cleaning means advantageously bears on a lower portion of said conductive surface below said separation zone but, in any event, is located outside of said separation zone. For example, where said conductive surface is generally cylindrical, said cleaning means may be located on the uppermost point of the generally cylindrical conductive surface with the feed of said species directed away from this region. However, it is preferred that said cleaning means bears upon the lowermost point of the generally cylindrical conductive surface.
In an arrangement in which two generally cylindrical conductive surfaces are appropriately juxtaposed, a single cleaning means may bear upon the lowermost point of one and the uppermost point of another.
Where said conductive surface is generally cylindrical, said drive means effects rotation of the generally cylindrical conductive surface in order to bring said conductive surface free from surface contamination into a position where it constitutes said separation zone.
Preferably the particle separator further comprises drive control means for indexing movement of said conductive surface through a plurality of zones, each zone in turn becoming said separation zone. For example, where said conductive surface is generally cylindrical indexed rotation of the generally cylindrical surface brings a different zone into the position where it can receive a feed of said species from said feeder means.
Advantageously said cleaning means includes cleaning control means for rotating same. Typically said cleaning control means causes said cleaning means to rotate concurrently with movement of said conductive surface, for example, to rotate concurrently with rotation of a generally cylindrical conductive surface. In a preferred form of the invention, said drive control means moves said conductive surface through at least several of said plurality of zones during an operational cycle before operation of said cleaning means is initiated. Thus, for example, feed may be applied to a plurality of zones located at different points around the circumference of a generally cylindrical conductive surface prior to initiating a cleaning cycle in which the conductive surface rotates through a single full rotation (i.e. through 360xc2x0) or through two or more full rotations in order to ensure that a surface free from surface contamination is produced.
Alternatively said drive control means moves said conductive surface from one zone to another with concurrent operation of said cleaning means. Thus there would be cleaning of a portion of said conductive surface during each indexed movement.
Advantageously said cleaning means is a cleaning or linish roll, drum, brush or cleaning pad.
Preferably said cleaning means is a roll brush or linish drum.
Surface contamination may arise in a number of forms but typically comprises ingrained particulate matter. The plates of electrostatic separators typically attract and become coated with non-conductive organic or inorganic film after hours or days of operation, and it has been found that removal of this type film improves performance of such separators.
Typically said feeder means meters said species onto said conductive surface at a predetermined rate.
Advantageously said feeder means comprises a feed plate whose angle of orientation is adjustable so as to be angled downwardly at between 25xc2x0 and 50xc2x0. The feed plate length may also be adjustable.
In a particularly preferred form of the invention the particle separator further comprises a second separation zone. The second separation zone has an electrode arrangement spaced apart therefrom in the manner of the separation zone described above and operates in like manner. In order to allow for two separation zones, the feeder means is adapted for dual feed of said species to each of the separation zones.
Typically the feeder means comprises oppositely directed feed plates whose angle of orientation and length is adjustable in the manner described above.
Advantageously the particle separator further comprises collection means for separately collecting conducting species and non-conducting species. Mid-range conductors may be collected separately from strongly conducting species and the non-conducting stream may be fed to a further particle separation stage if impure.
Typically said electrode arrangement comprises a single or multiple element high voltage electrode.
Preferably said high voltage electrode or one or more elements thereof is a dielectric electrode. The elements of a multiple element high voltage electrode may be separate electrodes or can be separate portions of a single electrode separated by non-conducting buffers.
Advantageously said dielectric electrode comprises:
a first glass substrate metallised on one of its surfaces to create an electrically conductive surface;
a high voltage lead in electrical connection with said electrically conductive surface; and
a dielectric material in electrically insulative abutment with said electrically conductive surface.
Typically said dielectric material is a second glass substrate.
In a particularly preferred embodiment of the invention a multiple element high voltage electrode is used, and said multiple element high voltage electrode comprises:
a primary electrode for inducing charge in said species spaced apart from at least an upper section of said first separation zone; and
a secondary electrode for lifting said species from said conductive surface once charged spaced apart from a lower section of said first separation zone.
It will be appreciated that a higher field density between said secondary electrode and said conductive surface than between said primary electrode and said conductive surface is advantageous since the larger the field the greater the tendency for charged particles to lift. However, even non-conducting particles will become charged if too high a field is employed between said primary electrode and said conductive surface, hence the field employed here must be of somewhat lesser intensity.
In a particularly preferred form of the invention said secondary electrode has a greater voltage applied thereto than said primary electrode, typically a 20% to 30% higher voltage.
Alternatively said secondary electrode may be positioned closer to said conductive surface than said primary electrode, and could include means for adjusting its position to vary the field between said secondary electrode and said conductive surface if desired. Said secondary electrode may also be orientated at a lesser angle to the vertical than said primary electrode in order to bring it close to a substantially vertical portion of said conductive surface when it is generally cylindrical in shape.
Alternatively a linear or curved single element high voltage electrode can be positioned above said first separation zone. If the electrode is curved, it is desirable that its curvature approximates that of the conductive surface.
According to a second aspect of the present invention there is provided a particle separator for the separation of particulate mixtures comprising species that exhibit difference in electrical conductivity, comprising:
a conductive surface including a separation zone over which said species move at a predetermined velocity;
feeder means for feeding species to said separation zone.
an electrode arrangement spaced apart from said conductive surface and capable of inducing charge in conducting species as they move across said separation zone and lifting said conducting species from said conductive surface once charged, said electrode arrangement comprising a primary electrode for inducing charge in said conducting species spaced apart from at least a first portion of said separation zone within which said particulate material makes contact with said conductive surface, and a secondary electrode for lifting said conducting species from said conductive surface once charged spaced apart from a second portion of said separation zone within which said conducting species lift from said conductive surface.
Advantageously there is a higher field density between said secondary electrode and said conductive surface than between said primary electrode and said conductive surface, and this may be achieved in the manner described above.
Advantageously the particle separator further comprises a second separation zone.
Advantageously a primary electrode for inducing charge in said conducting species is spaced apart from at least a first portion of said second separation zone within which said species makes contact with said conductive surface, and a second electrode for lifting said conducting species once charged is spaced apart from a second portion of said second separation zone within which said conducting species lift from said conductive surface.
The feeder means in this apparatus is typically as described above.
Advantageously the particle separator further comprises cleaning means for producing a conductive surface free from surface contamination. Preferred forms of the cleaning means are as described above.
Typically said cleaning means is spaced apart over said conductive surface from said feeder means.
Advantageously a particle separator in accordance with this aspect of the invention further comprises drive means for movement of said conductive surface relative to said electrode arrangement. Movement of said conductive surface relative to said electrode arrangement may be continuous or intermittent in the manner described for other aspects of the invention.
Typically said conductive surface is generally cylindrical in shape.
According to a third aspect of the present invention there is provided a dielectric electrode comprising:
a first glass substrate metallised on one of its surfaces to create an electrically conductive surface;
a high voltage lead in electrical connection with said electrically conductive surface; and
a dielectric material in electrically insulative abutment with said electrically conductive surface.
Typically said dielectric material is a second glass substrate.
According to a fourth aspect of the present invention there is provided a multi-stage particle separator for the separation of particulate mixtures comprising species that exhibit difference in electrical conductivity, comprising a particle separator as described above in operative association with a further particle separator or separators.
Advantageously, said further particle separator or separators is also as described above.
According to a fifth aspect of the present invention there is provided a process for the separation of particulate mixtures comprising species that exhibit difference in electrical conductivity, comprising the steps of:
1) providing a conductive surface including a separation zone over which said species move at a predetermined velocity, the conductivity of which reduces over time through surface contamination, which is spaced apart from an electrode arrangement capable of inducing charge in conducting species as they move across said conductive surface and lifting said species from said conductive surface once charged;
2) producing at least a portion of said conductive surface free from surface contamination; and
3) feeding said species onto said conductive surface free from surface contamination.